The present invention relates generally to computer networks and more particularly to computer networks of the type in which a plurality of geographically dispersed computer work stations are interconnected for interstation communications by a single electrically continuous bi-directional bus.
In recent years a number of different types of computer networks have been proposed and in some cases actually implemented wherein a plurality of geographically dispersed computer work stations are interconnected by a communications channel either for communication purposes, for collectively performing system tasks from different locations or for sharing a data storage facility at one of the computer work stations.
In many of these networks, especially those in which computer work stations are relatively close, the communications channel is a single electrically continuous bi-direction bus, such as a pair of twisted wires or a coaxial cable, which is used by the stations in a message or packet switching mode. Some of the advantages of the single bus type channel are its low cost, its overall simplicity and its capability for easily adding or removing computer work stations when desired or required. In the single bus network, all station-to-station communications, including status and control signals, take place over the bus. A message can be sent by any computer station connected to the bus and can be received by every other computer work station connected to the bus. Only one message, however, can be transmitted over the bus at one time. If two or more computer stations attempt to send messages over the bus at the same time, the messages will collide and be lost. In addition, the colliding messages may create a new message which is different from all of the original messages. The new message may be received by an intended receipient of one of the original messages or even by a computer station that was not intended to be a recipient of any of the original messages. Therefore, in the operation of a single bus network it is necessary that access to the bus by the computer work stations be controlled.
A number of different techniques have been devised for controlling access to the bus in a single bi-directional electrically continuous bus network.
In one prior art technique, known as the selection technique, a computer work station may access the bus only when it has been signalled that it is its turn to do. In one class of networks using this technique, the signals placing the computer work station in control are generated by a central controller unit and then sent to the various computer work stations either by a daisy chaining arrangement, by a polling arrangement or by an arrangement known as independent requests. In another class of networks using this technique, there is no central controller unit. Instead, the control logic is distributed evenly among the computer work stations. The control signals which are generated by the computer work stations are sent from one to the other by daisy chaining, by polling or by independent requests.
In another prior art technique known as the random access technique, a computer work station desiring to access the bus does not have to wait until it is placed in control but simply monitors the bus for activity. If there is activity, the computer work station waits. If there is no activity for a predetermined time interval, the computer work station assumes the bus is clear and transmits its message.
In still another prior art technique known as the reservation technique, a computer work station desiring to transmit a message places a request to do so and then receives a future reserved time during which it may transmit its message.
In my copending patent application Ser. No. 150,713, filed on May 19, 1980 and assigned to the assignee of this patent application and which patent application is incorporated herein by reference there is described a network in which control of the bus is distributed amongst the computer work stations in a manner which is contentionless and wherein operations of the network is not dependent on the operation of any particular computer work station.
In all of the above described networks, the computer work stations each include, in addition to the particular computer device, a transceiver for transmitting data signals from the computer to the bus and receiving data signals transmitted over the bus from other computer work stations and an interface circuit for interfacing the transceiver to the computer. In addition, each computer work station also includes a power supply for supplying operating power to the transceiver and the computer device. The power supplies receive their power from AC power sources which usually originate from a common power generating station and are tied to a common ground and the bus is usually tried to the same ground as the ground for the AC power sources.
One of the problems with this arrangement is that there is the potential for ground loops (i.e. from the common ground through one computer work station to the bus and then back to the common ground through another computer work station) which can cause a degradation of data transmitted over the bus, especially during transient conditions. The noise causing the degradation of the transmitted data may be electrostatic, electromagnetic, transients in the AC power lines and it may be common mode or differential mode. For example, electrostatic noise can be produced if a computer operator picks up an electrostatic charge by walking across a rug and then discharges that charge by touching the computer chassis or a metal desk on which the computer is mounted. An electromagnetic noise, on the other hand can be produced by a nearby motor, such as for example, a passing automobile. Common mode noise can be produced by a bolt of lightning striking both wires of the power lines simultaneously or both conductors on the bus simultaneously. Differential mode noise can be produced by a transient signal appearing on either one of the wires of the power lines (i.e. a sudden surge of power). In all cases, the noise will propogate through the bus as pulses and appear at the transceivers as pulses.
It has been suggested that ground loops might be avoided by using transformers to couple the computer work stations to the bus. An example of such an arrangement may be found in U.S. Pat. No. 4,063,220. The problem with using transformers as a coupling mechanism is that although they generally eliminate DC ground loops at relatively low frequencies, they do not prevent transient signals at high frequencies, such as 100 MHZ, from passing through the system. As is known, many computer networks operate at transmission reates of around 2 megabits per second and at these rates, or higher, it is conceivable that electrostatic discharges can have very high rise times going out well beyond the 100 MHZ region. Thus, as can be appreciated, transformer coupling does not completely eliminate ground loops.
Another technique that has been proposed has involved using capacitors as the coupling mechanism rather than transformers, the size of the capacitors determining the frequencies that would be passed. Since the capacitors have to be sized to pass the data signals, that is signals, around 2 MHZ, they would also pass signals greater than 2 MHZ and thus suffer the same shortcoming as transformers.
As can be seen, in order to reliably transfer data signals from one computer work station to another over an electrically continuous bus, an arrangement must be provided for coupling the computer work stations to the bus in a manner which will not have the potential for creating ground loops.
Accordingly it is an object of this invention to provide a new and improved computer network of the type in which a plurality of computer work stations are coupled to an electrically continuous bus.
It is another object of this invention to provide an arrangement for coupling a plurality of geographically dispersed computer work stations to an electrically continuous bus which does not have the potential for creating ground loops, even at high frequencies.
It is still another object of this invention to provide a technique for coupling a plurality of computer work stations to an electrically continuous bus in a manner which avoids the potential for ground and which does not involve the inductive or capacitive couplings.